Over the last decades, the field of optical lithography experienced rapid development allowing for semiconductor fabrication of increasingly smaller features. These developments require appropriate overlay metrology tools to allow accurate alignment of consecutively printed layers.
In addition, in order to enable decrease in pitch (distance between identical elements in a grating), advanced device designs involve novel attributes (e.g. large thicknesses, new materials). While crucial for the operation of the eventual device, these new designs often pose significant challenges to the overlay metrology, as they decrease the signal sensitivity to the overlay error.
Overlay measurement techniques have been developed based on proper design of overlay targets. This approach is disclosed for example in U.S. Pat. Nos. 6,801,315 and 6,974,962, both assigned to the assignee of the present application.
According to U.S. Pat. No. 6,801,315, the overlay measurement technique is performed by illuminating a sample with incident radiation, collecting the radiation response of the sample by an objective, splitting the collected radiation into two spatially separated radiation components, and directing the split radiation components towards at least one imaging plane along different optical channels characterized by optical paths of different lengths, respectively. Two image parts are thus obtained corresponding to the detected two split radiation components in the imaging plane(s), where each image part contains images of the two target structures. This enables determination of the relative distance between the two target structures.
According to U.S. Pat. No. 6,974,962, layers alignment in a multi-layer sample is controlled using two patterned structures located one above the other in two different layers, respectively, within a measurement site (e.g. a test site). The measurement site is illuminated with electromagnetic radiation thereby illuminating one of the patterned structures through the other, and a diffraction efficiency of radiation diffracted from the patterned structures is detected. The diffraction efficiency is indicative of a lateral shift between the patterned structures.